Project Description: Gas hydrate, a crystalline
form of gas (primarily methane) and water, occurs in continental slope margins
and arctic permafrost environments in concentrations that have the potential
to influence future energy supplies and the global carbon cycle. Integrated
Ocean Drilling Program (IODP) Expedition 311 was dedicated to understanding
the evolution of an entire gas hydrate system within the northern Cascadia
margin accretionary complex (fig. 1). Results
from this margin-perpendicular transect have challenged the simple model that
gas hydrate is concentrated where gas-charged fluids ascend into the gas hydrate
stability zone. Results from that program suggest local methane solubility,
fluid advection rates, and availability of suitable host material are also
important controlling factors.

The purpose of this project is to determine to what extent organic matter
controls the occurrence, distribution and fate of gas hydrate methane within
this accretionary margin. At the most fundamental level, the origin of
all methane in gas hydrate is buried organic matter. The mechanisms for
liberating methane from organic matter are either microbial processes occurring
in the upper 1000 m below the seafloor, high temperature pyrolysis at
greater depth where temperatures exceed ~120°C, or some combination of
both. The
age and depositional origin of the organic matter influence the reactivity
of the organic matter, which, in turn, influences the rates of methanogenesis. To
determine how the cycling of organic matter and the occurrence of gas hydrate
are related, organic geochemical data will be interpreted within the context
of proxies used for estimating gas hydrate sediment saturation. Stable
isotope (d13C and dD) data from reactive carbon pools in the gas hydrate and
dissolved fluid phases will be used to infer the methanogenic pathways (for
example, thermogenic-vs-microbial). Shifts in the isotopic signatures
along the transect will provide unique insight into the maturation of the organic
carbon reservoirs and its ability to generate enough methane to saturate the
pore fluids (a requisite for gas hydrate formation). Furthermore, the
concentration and isotopic composition of lipid biomarkers from microbes associated
with specific carbon cycles will be analyzed to constrain the role of chemotaxonomic
groups in specific carbon cycles.

Figure 1. Northern Cascadia margin, offshore Vancouver
Island (Canada). The presence of gas hydrate has been seismically
inferred beneath 50% of the continental margin. The study sites for
this project are the IODP Expedition 311 drilling transect and two cold
seeps (Bullseye vent and Barkley Canyon).